1. Field of the Invention
The present invention relates to a printed circuit board and a manufacturing method thereof.
2. Description of the Background Art
A flexible printed circuit board that can be bent is used for a movable element in electronic equipment. The flexible printed circuit board for example has wiring patterns made of a conductor formed on one surface of an insulating layer and a ground layer formed on the other surface. The wiring patterns are connected to various kinds of electronic components and transmit electrical signals between these electronic components.
In the flexible printed circuit board, various techniques have been devised in the ground layer to secure necessary flexibility.
FIG. 8 shows an example of a conventional flexible printed circuit board (see JP 2004-088020A). Hereinafter, the flexible printed circuit board will be abbreviated as “printed circuit board.”
FIG. 8(a) is a plan view of a signal wiring surface in the printed circuit board, and FIG. 8(b) is a plan view of a ground wiring surface in the printed circuit board. FIG. 8(c) is a sectional view of the printed circuit board taken along line A-A in FIGS. 8(a) and 8(b), and FIG. 8(d) is a sectional view of the printed circuit board taken along line B-B in FIGS. 8(a) and 8(b).
As shown in FIG. 8, the printed circuit board 30 has a base 33. A front layer (see FIG. 8(a)) having signal lines 31 and a ground line 32 is formed on one surface of the base 33, and a backside layer (see FIG. 8(b)) having ground planes 37a and 37b and ground lines 38 is formed on the other surface of the base 33.
A bend portion 35 is provided in a prescribed region of the printed circuit board 30, and flat portions 36a and 36b are provided to extend on both sides of the bend portion 35.
As shown in FIG. 8(a), at the front layer, the signal lines 31 and the ground line 32 are provided at prescribed intervals in the length-wise direction of the printed circuit board 30.
As shown in FIG. 8(b), at the backside layer, the ground planes 37a and 37b are provided at the flat portions 36a and 36b, and the ground lines 38 are provided at the bend portion 35. The ground lines 38 electrically connect the ground planes 37a and 37b. 
Here, as shown in FIG. 8(d), the ground lines 38 at the backside layer are provided in a staggered manner so as not to be opposed to the signal lines 31 at the front layer with the base 33 interposed therebetween.
In this way, the signal lines 31 at the front layer and the ground lines 38 at the backside layer are shifted so that they do not overlap. Therefore, if the printed circuit board 30 is bent at the bend portion 35, the printed circuit board 30 is less likely to be broken, and the signal lines 31 and the ground lines 38 are less likely to be disconnected.
If the wiring pattern transmits a high frequency signal, the characteristic impedance of the wiring pattern must be matched with the input/output impedance of an electronic component to be connected to the wiring pattern. If they do not match, a part of the electrical signal is reflected at the connection part between the wiring pattern and the electronic component, which degrades the transmission efficiency of the electrical signal.
In general, the characteristic impedance of the wiring pattern depends on the area of the region in which the wiring pattern and the ground layer are opposed to each other, and as the area of the region in which the wiring pattern and the ground layer are opposed to each other decreases, the characteristic impedance of the wiring pattern increases.
In the conventional flexible printed circuit board 30 shown in FIG. 8, the area of the region in which the signal lines 31 at the front layer and the ground layer at the backside layer (including the ground planes 37a and 37b and the ground lines 38) are opposed to one another is large at the flat portions 36a and 36b and small at the bend portion 35. Therefore, the characteristic impedance of the signal lines 31 greatly differ between the bend portion 35 and the flat portions 36a and 36b, which degrades the transmission efficiency of electrical signals.
In the conventional flexible printed circuit board 30, the flexibility at the bend portion 35 is secured while the flexibility of the flat portions 36a and 36b is not secured. Therefore, the region other than the bend portion 35 cannot be bent in use.
Meanwhile, it would be possible to form the ground lines 38 in a staggered manner with respect to the signal lines 31 at the flat portions 36a and 36b. 
However, in this case, the region in which the ground layer (ground lines 38) and the signal lines 31 are opposed to one another is reduced. Therefore, the characteristic impedance of the signal line 31 increases. In this way, if the input/output impedance of an electronic component is low, the characteristic impedance of the signal line 31 cannot be matched to the input/output impedance of the electronic component.